Integrated circuit dice are typically attached to a substrate that provides heat transfer and electrical connections. For high frequency circuits, heat transfer becomes especially important. In addition, for high frequency circuits, signal degradation due to reflections may limit the useful frequency range. Therefore, transmission line geometry becomes especially important.
A common method of electrically connecting the die to the substrate is wire bonding (or tape-automated bonding). Wire bonding consists of attaching wires to bonding pads on the die and to traces on the substrate. Flexible wire bonds provide strain relief to allow for differences in thermal expansion between the die and the substrate.
High frequency signals on an integrated circuit die and on the substrate are typically routed by microstrips or other transmission lines. Bonding wires between a substrate and a die form a discontinuity in the transmission line. This discontinuity causes signal degradation due to reflections.
For high frequency circuits, the substrate is typically a material with good thermal conductivity such as aluminum oxide or other ceramic material. The die is typically back bonded to the ceramic for thermal conduction through the back of the die.
FIGS. 1 and 2 illustrate a typical prior art system. FIG. 1 illustrates a top view of a corner of an integrated circuit wire bonded to traces on a substrate. FIG. 2 illustrates a cross section through the assembly illustrated in FIG. 1. In FIGS. 1 and 2, an integrated circuit 100 is wire bonded to traces (102 and 104) on a substrate 106. Trace 102 represents the top portion of a signal microstrip. A ground plane 112 on the bottom side of the substrate 106 forms the other conductor in the microstrip. Trace 104 represents a ground connection to the integrated circuit 100. A plated via 110 connects the ground trace 104 to the ground plane 112 on the bottom of the substrate. In general, traces leading to the integrated circuit 100 are so crowded that via 110 cannot be placed close to the integrated circuit 100.
For high frequency signals (for example signals in the GigaHertz range), trace 102 and the ground plane 112 comprise an effective transmission line. However, at the wire bond 108, there is a discontinuity in the microstrip geometry. At that point, there is no longer a transmission line. Instead of confined electric field lines between the trace 102 and the ground plane 112 below, there is a longer circuit through the integrated circuit requiring current flow through the plated via 110.
The present invention improves substrate thermal conductivity and reduces signal degradation relative to prior art die mounting methods.